Rigid internal fixation has been indicated for treatment of defects of the mammalian skeletal system for decades. Although external fixation such as plaster and splints have been used to stabilize the skeleton since ancient times, it was not until the emergence of steel wire in the nineteenth century that a practical method for treating non-isolatable bone fragments such as those found in craniomaxillofacial repair situations was developed.
A great advancement in skeletal fixation occurred in the late 1950s with the introduction of metallic plate systems. By securing the plate to the individual bone components with screws, this relatively simple device prevented fragment motility commonly encountered with wire-stabilized repair. These plates are generally sheets of metal that are fenestrated at various points along their lengths for fastening by screw. Over the years, metallic plate systems have become miniaturized and more biocompatible. Initially made from stainless steel, subsequent alloys, which include Vitallium® and titanium, were developed allowing for improved strength and rigidity. A panoply of geometric configurations is available to meet nearly every conceivable bone fixation need. The application of metallic materials has greatly improved aesthetic outcomes and has enabled earlier and more complete surgical reconstructions.
The search for improved fixation implants has lead to the development of a plethora of prostheses for utilization in surgical procedures, for example, fiber reinforced sheets to prevent hernias, bone plates to allow healing of bones after fracture and skull plates for use after cranial surgery. In particular, bone fixation plate and skull plate implants are utilized in a manner such that their placement may prevent bone fragment movement relative to the remainder of the bone. See, e.g., U.S. Pat. No. 3,741,205. The construction of these prostheses has historically been of some metal or metal alloy, e.g., surgical stainless steel, titanium, or Vitallium®. See, e.g., U.S. Pat. No. 6,344,042. These metal prostheses have the desired strength and rigidity to properly stabilize the area and allow the healing process to occur unimpeded by fragment and/or bone shifting. The stabilization may be external to the body, by use of a scaffold of rods and braces (see, e.g., U.S. Pat. No. 3,877,424) or alternatively, implanted internally and fastened to the bone via a securing means, such as cementing, medical staples, pins, nails, tacks, screws or clamps. See, e.g., U.S. Pat. Nos. 5,201,733; 6,454,770; 6,336,930.
Metal plates to be utilized as prostheses to immobilize bone fragments have the ability to be customized to fit the unique contours of each patient. Customization of the prosthesis is accomplished by twisting and bending the plates to fit the surgical site. Despite the utility of metallic plate systems, their use is not without problems. Multiple bending attempts may be required to achieve a desired fit, potentially fatiguing the metal. Furthermore, an extended customization and shaping process may lead to higher risk for the patient, due to a protracted period while under anesthesia, as well as increased opportunity for infection.
The consequences of long-term metal implants over a fifty to seventy year period are not known. Particles from these devices have been isolated in very distant organs such as the liver and the lung. Trace amounts of aluminum and nickel have been found in tissues surrounding implants thought to be composed of pure titanium. Metal plates have the drawback of remaining in place long after the healing process is complete, unless removed through a second invasive procedure. This intransience may be harmful where there is a need for continued bone growth and that growth is restrained by the implant, e.g., a child's skull must be capable of continued growth through development, and a metal skull plate, if left in place after cranial surgery, would interfere with developmental growth. Other postoperative complications from metallic plating systems include: visibility or palpability, hardware loosening with resulting extrusion (e.g. “screw backout”), temperature sensitivity to cold, screw migration and maxillary sinusitis, bone atrophy or osteopenia caused by stress shielding and corrosion, interference with radiographic imaging and radiation therapy, allergic reactions, intracranial migration in cranio-orbital surgery, and the possibility of causing growth restriction of the craniofacial skeleton on pediatric patients. Additionally, a metal prosthesis, if not removed after healing, may over time corrode or allow the leaching of metals to other locations of the body. For these reasons, the pursuit of other fixation technology has continued.
In order to overcome some or all of the drawbacks of metal implants, considerable attention has been given to the field of biodegradable (absorbable, resorbable) prostheses. These prostheses are capable of protecting the injury site, while still allowing the healing process to occur; however the resorbable nature of the prostheses allows the prostheses to remain in place only as long as would be needed to complete the healing process. The resorption of the implant obviates the need for a second surgical procedure to remove the implant, as might be required for a non-absorbable prosthesis, thereby reducing opportunity for infection or other complications. Additionally, any problems commonly associated with metal implants that may also be associated with resorbable implants, such as bone atrophy, would be transient, as the problem would not persist beyond the absorption of the implant.
The use of resorbable materials to form an implantable prosthesis is not new. See, e.g., U.S. Pat. No. 3,739,773. Bioresorbable internal fixation devices have been available for years principally as pins, plugs, screws, tacks and suture anchors. In 1996, the United States Food and Drug Administration approved the first bioresorbable internal fixation system for craniomaxillofacial indications (LactoSorb®, Walter Lorenz Surgical, Inc., Jacksonville, Fla.). Available in a variety of screws, panels and plate designs, the material is a non-porous amorphous copolymer of L-lactic and glycolic acid in a ratio of 82:18 and engineered to completely resorb in 9 to 15 months following placement. The material has manufactured fenestration points located throughout the device to allow for screw fixation to bone. Other resorbable materials now available commercially include Synthes® and MacroPoreFX™ fixation plates.
Prior art discloses that bioresorbable internal fixation plates can be manufactured by injection molding or compression molding techniques. For injection molding, a mold of the desired plate is first fabricated. The desired polymer is then heated significantly above its glass transition temperature until its viscosity is low enough to allow the polymer to flow. As this is occurring, a screw carries the molten polymer into the mold where it is allowed to cool below its glass transition temperature. The polymer is now solidified to the shape of the mold. The advantage to injection molding is that extremely intricate mold designs can be produced. One disadvantage of injection molding, however, is that the polymer may undergo a relatively long heat cycle, which breaks down the molecular weight of the polymer, thereby, affecting material strength as well as degradation characteristics. Another major disadvantage to injection molding is that there will usually be a certain amount of inherent stress within the plate due to freezing the polymer in place prior to it obtaining the orientation of lowest energy. Over time, or if the plate is heated prior to use, it will deform in order to relieve the stress in the part. In the case of heat application, the screw holes of the plate have been shown to become distorted causing some screw points to become unusable or preventing proper thread contact and alignment. Annealing may be used to help prevent this deformation from occurring. Annealing requires holding the plate in place while heating it above its glass transition temperature and waiting for the stress to relieve. This is an additional heating step and can lengthen a manufacturing process or further break down the molecular weight of the polymer.
Another method of thermally forming a polymer into a resorbable fixation plate is compression molding. Under this method, a mold is first produced and placed between hot platens. The two mold halves are separated and polymer is placed into the mold. Compressive pressure is applied to the mold and it is then heated above the glass transition temperature of the polymer. The polymer will eventually start to flow as the mold heats up and the material will take the shape of the mold. The advantage to this method is that the fixation plates incur very little molded-in stress because the polymer only has a relatively short distance to move. Disadvantages to this method include an even longer heating cycle than injection molding, a slow process that is difficult to use on a large scale, and a process that may require machining the holes into the fixation plate as a second operation.
The materials derived from ordinary thermal molding techniques (injection and compression molding) are not flexible at room temperature. Generally, the resorbable craniomaxillofacial products currently on the market are not deformable at room temperature and must be heated prior to implantation to adapt the device to the contours of the wound site. As the patients often vary in size, and because the bone surfaces are not flat, during implantation there exists a need to fit the prosthesis to the particular contours of each patient. Various techniques may be utilized to heat the prosthesis (e.g. exposure to hot air, immersion in hot liquid, exposure to radiation or exposure to some other heating medium) to a temperature above the glass transition point, but below the melting point; thereby making the prosthesis temporarily flexible, and allowing it to be fitted to an individual by bending, either by hand or with special bending tools. See, e.g., U.S. Pat. No. 5,290,281. After heating, the physician has only a limited amount of time, often just seconds, in which to accomplish the bending. Depending on the thickness of the plate, this period of malleability can be as short as two to three seconds causing the practitioner in many instances to expend considerable time to reheat and reshape the material several times while bending to achieve proper conformity. This additional time increases anesthesia requirements and operating room time and increases the potential of infection. See U.S. Pat. No. 6,332,884 for a prosthesis that turns to a clear solid while heated above its glass transition temperature, and reverts back to an opaque solid when cooled below the glass transition temperature, giving a visual indication to the physician about the status of the prosthesis' flexibility. The limited period of time available for bending of the prosthesis requires dexterity and care by the physician to create a shape that is adequate for use with each individual. The repeated heating, if necessary, to allow careful molding of the prosthesis adds to the time and complexity and cost of the procedure, further increasing the risk to the patient. Furthermore, if the prosthesis is not properly heated above the glass transition temperature as required for flexibility, and is bent while below the glass transition temperature, then the prosthesis will remain inflexible or be rather brittle, and likely develop cracks and/or micro-cracks upon bending.
In U.S. Pat. No. 6,221,075, there is disclosed a polymer tissue fixation device that may be deformed at room temperature. The ability of the polymer to be deformed without heating is made possible by an additional manufacturing step incorporated into the thermal molding techniques described above, where the polymer material is oriented with an uni- and/or biaxial solid state deformation process. The solid state deformation process orients the molecules of the polymer, so that room temperature bending is possible without substantial damage or breaking. The deformation process adds to the cost of manufacturing the tissue fixation device, adding to required labor and time of manufacture. In order to avoid unwanted bending or warping of the device while exposed to temperatures above the glass transition point but below the melting point, the device must be maintained at elevated temperature after deformation to allow stress relief of the polymer molecules. After the deformation step or stress relief step, any further modifications or machining, such as holes for fastening devices, must be created before use, such as by drilling.
Walter et al. in U.S. Pat. No. 6,203,573, disclose molded, biodegradable porous polymeric implant materials having a uniform pore size distribution. The materials can be molded into implants of any desired size and shape without loss of uniformity of pore size distribution. The material may be hand-shaped when warmed to body temperatures, and more preferably when warmed to at least about 45 degree C. and more preferably to at least about 50 degree C. Once at an elevated temperature, the implant material can be further hand-shaped to fit the defect into which they are placed and the desired shape for the regrown tissue.
A need, therefore exists for an internal fixation device that can be resorbed by the body over time, yet provide sufficient strength to prevent bone fragment motility over the healing period necessary for natural repair. Furthermore, the device must be capable of manual deformation at room temperature to fit the unique shape of each individual patient without the use of heat or chemical manipulation, wherein the deformation may occur by bending or application of a compressive force. Also the device must be capable of resisting the formation of micro-cracks caused by shaping, or distortions caused by the introduction of fastening techniques known in the art.
A prosthesis as described above would have the benefits of ease of use in surgery, along with the associated benefit for the patient of reducing the total time under anesthesia, and minimizing the risk of infection for the patient.
In U.S. Pat. Nos. 4,966,599 and 5,413,577, Pollock discloses a set of pre-formed bone plates, to be manufactured as a kit. The use of an individual bone plate, comprising one of many in the kit, will still require final shaping by bending or crimping of the plates while the patient is undergoing surgery. Pollock has taken an approach to minimize the amount of time required to customize the implant by manufacturing many plates, encompassing a plurality of generic shapes and sizes, such that only minor customization by bending of the appropriate prosthesis would be required. A multi-piece kit, such as described by Pollock, would necessarily result in waste as the unused sizes and shapes of the kit would be discarded as not being appropriate for the specific patient's needs.
In U.S. Pat. No. 4,186,448, Brekke describes the use of a porous body, made of biodegradable material, to fill or cover a bone void. This material includes interconnected, randomly positioned, randomly shaped and randomly sized voids extending throughout the mass of the body member. The voids promote the penetration of blood into the prosthesis and aid healing through the facilitation of tissue and/or bone growth into the prosthesis. The prosthesis as described promotes tissue ingrowth and is replaced by new bone upon resorption.
The use of a resorbable prosthesis that serves as a barrier to cell permeability, while allowing bone wound or void healing is disclosed by Hayes et al. in U.S. Pat. No. 6,031,148, and also by Brekke et al. in U.S. Pat. No. 5,855,608. Hayes' prosthetic material serves as a pliable barrier to cells, acting to prevent soft tissue growth in areas where bone growth is desired. The Hayes patent discloses the pliable prosthesis having a matrix that is sufficiently open to allow infiltration of blood and subsequent interconnection of ingrowing tissue through the open spaces. Brekke discloses a resorbable implant that is capable of serving as a barrier to isolate one form of tissue (i.e. bone) from another form of tissue (i.e. soft tissue). Once implanted, the barrier prosthesis would serve to protect a void or wound in one tissue (the bone) from encroachment by the adjoining (soft) tissue, which would otherwise grow unobstructed into the void, precluding the void from proper repair with the original type of (bone) tissue.
In EPA 0 274 898, Hinsch discloses a foam-like, resorbable, plastic material, incorporating textile reinforcing elements made from resorbable plastic embedded in an open-cell plastic matrix, the open-cell matrix formed by a vacuum, freeze drying process. The application disclosure includes tables demonstrating how the tensile strength of the implant increases upon addition of the textile reinforcing elements. This increased tensile strength results in an implant that is more resistant to pulling and tearing forces. One of the stated objectives of the invention is to have an open cell structure to permit the growing in of cells and blood vessels, yet still retain adequate tensile strength to serve as an implant. According to the disclosure, the pores must be of sufficient average size to allow the ingrowth of cells and blood vessels.
The aforementioned application EPA 0 274 898 (Hinsch), as well as U.S. Pat. No. 4,186,448 (Brekke), U.S. Pat. No. 6,031,148 (Hayes) and U.S. Pat. No. 5,855,608 (Brekke) disclose resorbable prostheses used to fill or cover tissue voids, relying on the formation of new bone and tissue within the implant. None of these disclosures anticipate the need of a prosthesis that conforms to a surgical site via collapse of pores and is utilized to anchor tissue fragments together.
The use of composites in prostheses has been used to improve both mechanical and biological properties. In PCT application WO 86/00533, for example, Leenslag discloses a composite of fiber material, which may or may not be biodegradable, incorporated in a porous matrix of a biodegradable organic polymer material. The material as described by Leenslag is suitable for repair or replacement of torn bony material, the term bony material as used therein referring to a damaged meniscus, not to a wound in a bone as contemplated by the subject invention. The design of the prosthesis is such that it requires rapid ingrowth of tissue and vessels as part of its function.
Bowman et al., in U.S. Patent Application Publication No. US 2002/0127265 A1, describes a biocompatible tissue repair stimulating implant or “scaffold” device. The application discloses an implant that facilitates cellular ingrowth, by the open cell foam structure of the polymer, as well as by the delivery of tissue growth stimulating compounds as biological agents within the device. The implant as described may incorporate at least one layer of a mesh or weave of fibers to lend mechanical support to the device, in order to enable the device to be handled in the operating room prior to and during implantation, to enable the implant to resist suture pull through, and to enable the foam device to withstand stresses placed upon it while implanted. This compound implant of foam and fiber reinforcement is implanted with the aim of encouraging tissue ingrowth into the implant, such that as the device is reabsorbed, tissue growth penetrates into the device.
Both the Leenslag patent and the Bowman application are for devices operating in a manner similar to the aforementioned devices disclosed by EPA 0 274 898, U.S. Pat. No. 4,186,448, U.S. Pat. No. 6,031,148 and U.S. Pat. No. 5,855,608, in that they function as a void filler or tissue replacement.
An implantable, bioresorbable membrane used to allow healing of a tissue defect site is disclosed by Yoon et al. in U.S. Pat. No. 5,948,020. As described therein, the membrane serves to isolate a tissue defect site from encroachment by adjoining tissue while allowing the wound to heal. The implant may also incorporate woven or knitted fabric made of bioresorbable fibers as a support embedded in a bioresorbable porous polymer matrix. To achieve sufficient malleability and dimensional stability, as well as to avoid prior art, the patent discloses an implant whose surfaces have been heated above the glass transition temperature to 150 C and forcing a plate with 20 protrusions/cm2 into the already porous device (the embossing step).
Vyakarnum in U.S. Pat. No. 6,306,424 discloses an implant useful as a tissue scaffold, for repair or regeneration of tissue having architectural gradients (e.g. bone, cartilage, and skin), wherein the implant relies on gradients that mimic the histologic pattern of the tissues into which it is implanted.
There exists a need for an implantable, bioabsorbable prosthesis that is capable of being customized quickly, effectively and easily for the particular needs of each patient. The prosthesis should be capable of being fastened quickly and easily by a variety of fastening methods known in the art, including the use of staples, sutures, adhesives, nails, tacks, pins or clamps. The prosthesis must allow customization by responding to bending and compressive forces by smoothly bending and holding the desired shape, rather than cracking or breaking suddenly. The customization process should be simple, without requiring specialized tools or heating, thereby saving time and cost in the operation, as well as minimizing risk to the patient from prolonged exposure to infection and anesthesia. The prosthesis should allow the physician to make the customization in situ, while in the surgery suite, even while partially implanted. Furthermore, the absorbable prosthesis should be rigid enough to serve to isolate and protect the tissue from shifting. The prosthesis should be capable of being fully absorbed after the healing process has completed.
It is the intent of this invention to overcome these and other shortcomings of the prior art.